It is known, for example from German patent document 3,610,221, for a cordless electrical tool (by which term is meant any appliance such as a drill, hair dryer, blender, or the like) to have a pair of sockets or seats that can each receive a respective battery pack. An indicator for the charge level of each battery is provided as well as a switch that allows the motor of the tool to be disconnected from one of the batteries when its charge level is too low and connected to the other. In this manner it is possible to continue to operate at the most efficient high-charge level of the batteries and to avoid excessively depleting them.
Such an arrangement is rarely, however, capable of providing sufficient power for a heavy-duty tool such as a hammer drill. Even though it is known to connect batteries in series, in which case the voltages are added to one another, such a connection has the considerable disadvantage that it is possible to run one of the batteries down so much that it is ruined. In other words the tool might continue to run even though one of its series-connected batteries is fully discharged, in which case this discharged battery might be so excessively drained that it cannot be recharged.